Radiation therapy has been employed to treat tumorous tissue. In radiation therapy, a high energy beam is applied from an external source towards the patient. The external source, which may be rotating (as in the case for arc therapy), produces a collimated beam of radiation that is directed into the patient to the target site. The dose and placement of the dose must be accurately controlled to ensure that the tumor receives sufficient radiation, and that damage to the surrounding healthy tissue is minimized.
Sometimes, before a radiation therapy is performed, the target region of the patient is imaged using a CT system for diagnostic purpose, or for treatment planning. For the case in which the target region moves in a periodic motion (e.g., due to breathing), the CT system may be used to determine volumetric images of the target when the target is at different breathing states, so that the volumetric images may be played back as a video stream. For such purpose, projection images of the target when the target is at different breathing states are acquired, and a breathing monitoring device is used to determine breathing states of the patient as the CT system acquires the projection images. After the imaging session, the projection images are then sorted according to the recorded breathing states of the patient when the corresponding projection images are acquired. The breathing monitoring device is required to track the breathing states accurately. The tracked breathing states cannot be too coarse (e.g., they cannot merely indicate whether the patient is at an inhale state or an exhale state) because otherwise, the resulting video stream would be too coarse for diagnostic and treatment planning purposes.
Also, sometimes during a radiation therapy, the patient may also be undergoing breathing motion. In such cases, it may be desirable to determine positions of a moving target such that a radiation beam may be adjusted accordingly to treat the target. Existing technique for determining a three dimensional position of an object requires simultaneous imaging of a target by two or more imaging systems. In such cases, the 3D position of a target cannot be determined when only one imaging system is available. Also, existing systems that are designed to use two imaging systems simultaneously cannot use images acquired at different times to determine a target position. In addition, existing systems that are designed to use two imaging systems simultaneously require the frame rates of the two imaging systems be the same and that they be synchronized. In such cases, the existing systems cannot determine a position of the target at all times if the two imaging systems have different frame rates, or if the image acquisitions by the two imaging systems are not synchronized.